A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
Efforts to keep up with the well-known Moore's law, which aims that the density of components in an integrated circuit should quadruple every three years, drive continual improvements in lithographic apparatus. The critical dimension of features that can be printed using a lithographic apparatus is proportional to the wavelength of the exposure radiation used divided by the numeric aperture (NA) of the projection system of the apparatus. Therefore there is a trend to provide projection systems with ever increasing numeric aperture. However, as the numeric aperture of the projection system increases, so does its size and complexity. Also, the depth of focus (DOF) is reduced, making imaging harder, particularly of difficult patterns such as contact hole arrays.